1. Field of the Invention
The present invention relates to a magnetic recording head that has a spin torque oscillator suitable for data storage of high recording density, high recording capacity, and a high data transfer rate. The present invention also relates to a magnetic head assembly and a magnetic recording/reproducing apparatus.
2. Related Art
In 1990's, there were dramatic increases in recording density and recording capacity of an HDD (hard disk drive), as MR (Magneto-Resistive effect) heads and GMR (Giant Magneto-Resistive effect) heads were put into practical use. However, the recording density increase rate temporarily became lower in the beginning of the 2000's, since the problem of heat fluctuations of magnetic recording media rose up to the surface. Recently, the HDD recording density has been increasing about 40% per annum, as the vertical magnetic recording that was more suitable for high-density recording in principle than in-plane magnetic recording were put into practical use in 2005.
In the latest recording density demonstration experiment, the 250 Gbits/inch2 level has been reached. If the progress continues at this rate, recording density of 1 Tbits/inch2 is expected to be reached around the year 2012. However, achieving such high recording density is considered not easy even by a vertical magnetic recording method, as the problem of heat fluctuations will surface again.
As a recording method to solve the above problem, a “high-frequency field assist recording method” has been suggested. By the high-frequency field assist recording method, a high-frequency magnetic field that is much higher than a recording signal frequency and is close to the resonance frequency of the magnetic recording medium is locally induced. As a result, the magnetic recording medium resonates, and the coercive force Hc of the magnetic recording medium having the high-frequency magnetic field induced therein can be greatly reduced. Therefore, by overlapping the recording magnetic field with the high-frequency magnetic field, magnetic recording can be performed on a magnetic recording medium that has much higher coercive force Hc and much greater magnetic anisotropic energy Ku.
As a technique for generating a high-frequency magnetic field, a technique that involves a spin torque oscillator has been suggested (see U.S. Patent Application Publication Nos. 2005/0023938 and 2008/0019040, for example). According to the techniques disclosed in U.S. Patent Application Publication Nos. 2005/0023938 and 2008/0019040, the spin torque oscillator includes a spin injection layer, a nonmagnetic layer, a magnetic layer, and a pair of electrode layers that sandwich those layers. When a direct current flows into the spin torque oscillator through the pair of electrode layers, the magnetization of the magnetic layer ferromagnetically resonates by virtue of the spin torque generated from the spin injection layer. As a result, a high-frequency magnetic field is generated from the spin torque oscillator.
Since the size of the spin torque oscillator is several tens of nanometers, the generated high-frequency magnetic field locally exists at a distance of several tens of nanometers from the spin torque oscillator. Further, the in-plane components of the high-frequency magnetic field can efficiently cause a vertically-magnetized magnetic recording medium to resonate, and the coercive force of the magnetic recording medium can be greatly reduced. As a result, high-density magnetic recording is performed only on the region where the recording magnetic field generated from the main magnetic pole is overlapped with the high-frequency magnetic field generated from the spin torque oscillator. Accordingly, it becomes possible to use a magnetic recording medium having high coercive force Hc and large magnetic anisotropic energy Ku. Thus, the problem of heat fluctuations to be caused at the time of high-density recording can be avoided.
In a case where the spin torque oscillator is located close to the main magnetic pole in the above structure, however, a large magnetic field is generated from the main magnetic pole toward the spin torque oscillator when writing is performed. The write magnetic field generated from the main magnetic pole has its direction reversed between positive and negative directions, depending on the writing direction. As a result, the oscillation characteristics of the spin torque oscillator are greatly changed with the change in the write magnetic field generated from the main magnetic field, and it becomes difficult to generate a stable high-frequency magnetic field.
As will be described later, due to the magnetic field generated from the main magnetic pole, the oscillation frequency of the spin torque oscillator and the current density required for oscillation are increased, and it becomes difficult to maintain the reliability of the spin torque oscillator.